A message containing letters from A-Zis being encoded to numbers using the following mapping:

'A' - > 1

'B' - > 2
...
'Z' - > 26

Given an encoded message containing digits, determine the total number of ways to decode it.

For example,
Given encoded message"12", it could be decoded as"AB"(1 2) or"L"(12). The number of ways decoding"12"is 2.

Solution: dp[i]:前i个字母的组合数。2种情况：当前位数 > 0时，dp[i] += dp[i - 1]；上一位数不为0且合起来 < 26时，dp[i] += dp[i - 2].

dp[0] = 1, dp[1] = 1(第一个字母不为0)

    public int numDecodings(String s) {
        if (s == null || s.length() == 0 || s.charAt(0) == '0') {
            return 0;
        }
        int[] dp = new int[s.length() + 1];
        dp[0] = 1;
        dp[1] = 1;
        for (int i = 2; i < dp.length; i++) {
            int one_digit = s.charAt(i - 1) - '0';
            if (one_digit > 0 && one_digit <= 9) {
                dp[i] += dp[i - 1];
            }
            int last_digit = s.charAt(i - 2) - '0';
            if (last_digit == 0) {
                continue;
            }
            if (last_digit * 10 + one_digit <= 26) {
                dp[i] += dp[i - 2];
            }
        }
        return dp[s.length()];
    }